If your thinking about doing a 7.62 build then I would recommend looking at the Grendel. The Grendel has almost the same ballistics as the 7.62 at short range and better ballistics past 300 yards. It's the same overall length as the 5.56 so it fits in the AR-15 platform instead of the AR-10 platform the 7.62 requires. AR-10 parts are more expansive and seems to be harder to get and less to choose from.

If you may be interested head over to http://www.65grendel.com/forum/forum.php and look around. Bill Alexander who developed the Grendel round is fairly active on the forums as well as other manufacturers who specialize in the Grendel round.